Not Applicable
Not Applicable
This invention relates generally to feedthrough nulling systems, and more particularly to systems for nulling drive signal feedthrough in sensors.
Coriolis force sensors such as tuning-fork gyroscopes are known in the art. These sensors can be fabricated using MEMS (microelectromechanical systems) techniques, by way of example. Micromachined Coriolis force sensors generally include a pair of proof masses which are positioned on opposite sides of an input axis, and which are connected to a substrate by support flexures. The flexures support the proof masses so as to allow the proof masses to vibrate in a nominal plane that includes the input axis. Time varying drive signals are provided by way of drive electrodes to impart an in-plane, anti-parallel vibratory motion to the proof masses. Typically, the frequency of the drive signals is set at or near the mechanical resonant frequency of the sensor assembly.
Position sensitive in-plane pick-off electrodes are usually provided for sensing in-plane displacements of the proof masses caused by the vibratory motion. The pick-off electrodes are coupled to a feedback gain control circuit, which controls the amplitude of the drive signals, and thus the amplitude of vibration of the proof masses. For stable and predictable gyroscope performance, the amplitude of vibration of the proof masses is preferably maintained at a predetermined, constant level.
Upon rotation of the sensor assembly about the input axis, the vibrating proof masses produce anti-parallel, out-of-plane deflections, caused by Coriolis forces which arise in response to the rotation. Out-of-plane sensing electrodes are provided in order to detect the out-of-plane deflections. Due to the nature of the Coriolis force, whose magnitude depends on the rotational rate of the sensor assembly, the amplitudes of the Coriolis deflections are proportional to the rotational rate of the sensor about the input axis. The sensing electrodes thus generate a signal indicative of the rotational rate of the gyroscope.
The performance of sensors such as the tuning fork gyroscope discussed above is adversely affected by a feedthrough of the drive signals into the sensor output, resulting from a coupling of signals from the drive electrodes into the out-of-plane sensing electrodes. Drive feedthrough affects measurements by both the out-of-plane sensing electrodes and the in-plane pick-off electrodes, thus limiting gyroscope performance.
It is an object of this invention to provide a system for significantly reducing drive feedthrough, thereby improving sensor performance.
The present invention relates to a system for nulling drive feedthrough error in sensors such as tuning fork gyroscopes. By reducing the undesirable effects of drive feedthrough, the present invention reduces one of the largest electronic sources of error in tuning fork gyroscopes. Further, the technique of the present invention has general applicability, and can be used to null other forms of drive feedthrough.
In one embodiment, the present invention relates to a system for nulling feedthrough error in a sensor having first and second drive electrodes which impart vibratory motion to first and second proof masses in response to first and second opposite phase drive signals, and having capacitances defined between the drive electrodes and their associated proof masses. The present invention is predicated in part on the recognition that capacitance mismatch between the drive electrodes and the associated proof masses is a dominant cause of drive feedthrough in Coriolis force sensors.
In accordance with the present invention, a differential drive amplitude is adjusted in order to compensate for drive capacitance mismatch. Compensation is accomplished by measuring the capacitance mismatch between the first and second drive electrodes and their associated proof masses, and nulling the measured capacitance mismatch by adjusting the drive signals. The capacitance mismatch can be measured by detecting signals that are proportional to the capacitance mismatch. In one embodiment, an off-frequency artifact is detected in order to obtain a measure of the drive capacitance mismatch.
In accordance with the present invention, a sensor for detecting a rotational rate about an input axis includes a substrate, and first and second proof masses positioned on opposite sides of an input axis. The proof masses are flexurally coupled to the substrate so as to permit an in-plane vibratory motion of the masses in a nominal plane including the input axis, and an out-of-plane vibratory motion in a direction perpendicular to the nominal plane. Each proof mass includes a electronically conductive region.
A driver includes first and second drive electrodes opposite the electrically conductive regions of the first and second proof masses. First and second capacitances are defined between the first drive electrode and the first proof mass, and between the second drive electrode and the second proof mass, respectively. The first and second drive electrodes are adapted to receive first and second a.c. opposite phase electrical drive signals for electrostatically driving the first and second proof masses to effect the anti-parallel vibration in the nominal plane. The frequency of the drive signals is preferably set to be substantially equal to a fundamental frequency corresponding to a mechanical resonant frequency of the sensor.
First and second sensing electrodes are provided for detecting the out of plane vibratory motion perpendicular to the nominal plane, arising from the Coriolis force that acts upon the proof masses when the sensor assembly rotates about the input axis. Each sensing electrode is fixedly positioned with respect to the substrate, and is disposed opposite the conductive region on a respective one of the first and second proof masses. The sensor is preferably micromachined, i.e. the proof masses, the drive electrodes, and the sensing electrodes are made from an etched silicon structure.
A circuit generates an output signal corresponding to the separation between the sensing electrodes and the conductive regions of the proof masses opposite. The output signal is representative of the rotational rate of the sensor about the input axis. Means are provided for adjusting the relative amplitudes of the first and second a.c. drive signals, whereby the ratio of the amplitudes is proportional to the ratio of the first and second capacitances. The adjustment of relative amplitudes of the drive signals substantially nulls drive feedthrough in the output signal caused by a mismatch in the first and second capacitances. In particular, when the drive signals have opposite polarities, and have a magnitude ratio equal to the reciprocal of the ratio of the corresponding capacitances, drive feedthrough from drive capacitance mismatch is substantially nulled.
The circuit may include first and second circuits. The first circuit is operative to provide an indicative signal proportional to a capacitance mismatch between the first and second capacitances. The second circuit is operative to measure the capacitance mismatch using the indicative signal, and to compensate for the measured capacitance mismatch by adjusting the first and second drive signals so as to substantially null drive feedthrough.
The first circuit may include a sense preamplifier connected to the proof masses. The output from the sense preamplifier contains a signal that is indicative of the mismatch between the first and second capacitances. The indicative signal can be demodulated in the second circuit to obtain a measure of the drive capacitance mismatch. The indicative signal may be a separate tracer signal that has been added to the drive signals. Alternatively, the indicative signal may be an inherent off-frequency component of the drive signal, in which the sensor resonant frequency and its harmonics have been suppressed.
The second circuit may include an adjustment device responsive to a controller for effecting the ratio of amplitudes, and a servo loop for adaptively effecting the ratio of amplitudes. In one embodiment, the adjustment device includes first and second variable gain amplifiers (VGAs) interposed between the servo loop and the first and second drive electrodes. The VGAs are operative to adjust the differential amplitude of the drive signals in response to the controller.
The second circuit may include means for demodulating the indicative signal from the first circuit so as to obtain a DC signal proportional to the capacitance mismatch. In one embodiment, the means for demodulating the indicative signal may include a multiplier, a low pass filter for removing modulation components from the indicative signal, and a controller. The demodulated signal may be passed through the controller to provide a control signal. The control signal may be split into first and second control signals of opposite phase, which control the first and second VGAs, respectively.
The present invention also relates to a method for nulling drive feedthrough in a sensor having first and second drive electrodes which impart vibratory motion to first and second members in response to first and second drive signals, and having first and second capacitances defined between the first and second drive electrodes and their associated proof masses. A mismatch between the first capacitance and the second capacitance is measured. Drive feedthrough caused by the mismatch is nulled by compensating for the measured capacitance mismatch. The relative amplitudes of the first and second drive signals is adjusted in order to compensate for the measured capacitance mismatch.